Semiconductor device

ABSTRACT

An ohmic contact between an electrode and a semiconductor layer is more stably formed and an electrical contact resistance between them is further reduced. 
     A semiconductor device comprises a semiconductor layer  103  composed of an oxide semiconductor material containing indium, an ohmic electrode  107  provided on the semiconductor layer  103  and having an ohmic contact with the semiconductor layer  103 , and an intermediate layer  106  provided between the semiconductor layer  103  and the ohmic electrode  107 , wherein the intermediate layer  106  includes a first region  106   a  whose indium atomic concentration is greater than that of an interior of the semiconductor layer  103  and a second region  106   b  whose indium atomic concentration is less than that of the first region.

TECHNICAL FIELD

This invention relates to a semiconductor device provided with anelectrode that forms an ohmic contact with a semiconductor layer.

BACKGROUND ART

Electrically conductive oxide semiconductors have in recent yearsattracted attention as materials for forming optically transparent,transparent electrodes or the active (channel) layer of thin-filmtransistors (abbreviation: TFT) or other semiconductor devices.Thin-film transistors that utilize an oxide semiconductor as the activelayer are being actively applied to flat panel display devices likeliquid crystal display (abbreviation: LCD) devices and organicelectro-luminescent (abbreviation: EL) devices. Moreover, transparentelectrodes containing an oxide semiconductor as a component have beenapplied to flat display panels and other flat panel display devices, aswell as to touch panels.

In these fields of industrial application of electrically conductiveoxide semiconductors, in order to reduce the RC delay of signaltransmission, ohmic electrodes composed of metal materials of highelectrical conductivity and low electrical resistance are used for theinterconnections and electrodes of the oxide semiconductor. The priorart uses, for example, aluminum (element symbol: Al), aluminum alloy,molybdenum (element symbol: Mo), and the like. Further, there have beenproposed electrodes and interconnectors formed by laminating dissimilarmetal layers of alloys of titanium (element symbol: Ti) or aluminum andsilicon (element symbol: Si). Recently, moreover, a technique has beentried that forms an oxide semiconductor ohmic electrode from copper(element symbol: Cu), which has low electrical resistance.

For example, regarding a thin Film Transistor (abbreviation: TFT) usedin a liquid crystal display device (abbreviation: LCD), technologies areavailable for using copper alloy to form the ohmic electrodesconstituting a source electrode and a drain electrode, as well asinterconnectors (see Patent document 1 to 7). In particular, Patentdocument 5 teaches that by using a copper alloy obtained by adding asuitable additive element to copper, oxidation of the copper isinhibited by a metallic oxide film formed by the additive element, sothat ohmic electrodes of low electrical contact resistance and copperinterconnectors of small RC delay are furnished.

It has been suggested that manganese (element symbol: Mn) is preferableas the additive element (see Patent document 5). Non-patent document 1teaches a technique for forming a copper electrode on a thin-filmtransistor that uses an electrically conductive oxide semiconductor asan active (channel) layer. Specifically, copper-manganese (Cu—Mn) alloyis used when forming a copper electrode on a thin-film transistor usingamorphous gallium oxide-indium-zinc (Ga—In—Zn—O) as the active layer.

PRIOR ART DOCUMENTS Patent Documents

-   Patent document 1: Japanese unexamined patent publication    2005-166757-   Patent document 2: Japanese unexamined patent publication 2002-69550-   Patent document 3: Japanese unexamined patent publication    2005-158887-   Patent document 4: Japanese unexamined patent publication 2004-91907-   Patent document 5: WO 2006/025347A1-   Patent document 6: Japanese Patent 3302894-   Patent document 7: Japanese unexamined patent publication    2004-163901

Non-Patent Document

-   Non-patent document 1: P. S. Yun, J. Koike, The 37th Spring Meeting,    2010; The Japan Society of Applied Physics and Related Societies    (March 17 to Mar. 20, 2010), Presentation No. 17a-TL-4,    “Microstructure analysis of the reaction interface of    Cu—Mn/In—Ga=Zn—O thin films”

Making the active layer of oxide semiconductor, whose electron mobilityis about tenfold that of an amorphous silicon which is the constituentmaterial of the active layer of a conventional thin-film transistor,would enable configuration of a thin-film transistor capable ofhigh-speed operation. In addition, if it should be possible to stablyform the ohmic electrodes and gate electrode of copper having lowelectrical resistance, it would be possible to enhance definition offlat displays.

However, the actual situation is that no technology has been adequatelydeveloped to stably form on an oxide semiconductor ohmic electrodes madeof copper of low electrical contact resistance. The prior art has notyet thoroughly determined a copper ohmic electrode/oxide semiconductorlayer interface contact structure which is for producing an ohmicelectrode of low electrical contact resistance when a copper-manganesealloy, for example, is used to form a copper source electrode or a drainelectrode on an oxide semiconductor active layer containing indium(element symbol: In), for example.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention was made to solve the aforesaid problems of theprior art and has as its object to provide a semiconductor devicewherein ohmic contact can be stably formed between an electrode and asemiconductor layer to enable still further reduction of electricalcontact resistance therebetween.

Means for Solving the Problems

With regard to an electrically conductive oxide semiconductor layer,this invention i) presents a contact structure at the interface betweena copper ohmic electrode and an oxide semiconductor layer, the copperohmic electrode giving rise to an ohmic electrode made of copper andhaving low electrical contact resistance, and provides a semiconductordevice furnished with an ohmic electrode having the contact structure.

Means (1) to (16) for achieving the object of the present invention arelisted below.

(1) A semiconductor device comprising: a semiconductor layer composed ofan oxide semiconductor material containing indium; an ohmic electrodeprovided on the semiconductor layer and having an ohmic contact with thesemiconductor layer; and an intermediate layer provided between thesemiconductor layer and the ohmic electrode,

wherein the intermediate layer includes a first region whose indiumatomic concentration is greater than that of an interior of thesemiconductor layer and a second region whose indium atomicconcentration is less than that of the first region.

(2) In (1), the semiconductor device wherein the first region is locatedin contact with the semiconductor layer and the second region is locatedin contact with the ohmic electrode.

(3) In (1) or (2), the semiconductor device wherein the first region iscomposed of crystal grains containing indium (In).

(4) In any of (1) to (3), the semiconductor device wherein the secondregion is composed of amorphous material.

(5) In any of (1) to (4), the semiconductor device wherein the thicknessof the intermediate layer where the first region and the second regionmeet is 3 nm or more and 30 nm or less.

(6) In any of (1) to (5), the semiconductor device wherein the first andsecond regions are constituted to contain an oxide of a metal formingthe ohmic electrode.

(7) In (6), the semiconductor device as recited in claim 6, wherein themetal forming the ohmic electrode is a metal with a free energy of oxideformation of a lower value than indium.

(8) In (6) or (7), the semiconductor device wherein the metal formingthe ohmic electrode contains at least one among manganese (Mn),molybdenum (Mo), and titanium (Ti).

(9) In (8), the semiconductor device wherein the metal forming the ohmicelectrode contains manganese.

(10) In (9), the semiconductor device wherein an electrovalence of themanganese increases from the second region toward the first region.

(11) In (9) or (10), the semiconductor device wherein an atomicconcentration of the manganese exhibits maximum value in the secondregion and is lower in the first region than in the second region.

(12) In (8), the semiconductor device as recited in claim 8, wherein themetal forming the ohmic electrode contains titanium.

(13) In (12), the semiconductor device as recited in claim 12, whereinthe electrovalence of the titanium increases from the second regiontoward the first region.

(14) In any of (1) to (13), the semiconductor device a wherein oxygenconcentrations of the first and second regions are lower than an oxygenconcentration of the semiconductor layer.

(15) In any of (1) to (14), the semiconductor device wherein the ohmicelectrode is formed of a copper alloy whose main constituent element iscopper.

(16) In (15), the semiconductor device wherein the second regioncontains more copper than the first region.

Effect of the Invention

In the present invention, the intermediate layer is constituted toinclude the first region whose indium atomic concentration is greaterthan that of the interior of the semiconductor layer and the secondregion whose indium atomic concentration is less than that of the firstregion. An ohmic electrode of low electrical contact resistance can beobtained because the indium-concentrated first region acts as a highconductivity layer.

Further, since the second region of the intermediate layer is given alower atomic concentration of indium than the first region, the secondregion exerts and effect of inhibiting intrusion into the electrode ofindium that is a constituent of the oxide semiconductor layer, wherebyan ohmic electrode of low electrical resistance containing substantiallyno indium can be stably formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a multilayer structure of afirst embodiment prior to heat treatment.

FIG. 2 is a cross-sectional TEM image of an interface vicinity afterheat treatment in the first embodiment.

FIG. 3 is diagram showing an energy intensity dispersion of elements inthe interface vicinity after heat treatment in the first embodiment asdetermined using an X-ray energy dispersive spectrometer (EDS).

FIG. 4 is a diagram showing the energy intensity dispersion of elementsin the interface vicinity after heat treatment in the first embodimentas determined using an electron energy loss spectrometer (EELS).

FIG. 5 is a diagram showing a binding energy of elements in an interiorof an intermediate layer after heat treatment of the first embodiment.

FIG. 6 is a diagram showing current-voltage characteristics in anelectrode structure obtained in the first embodiment.

FIG. 7 is a sectional TEM image of an interface vicinity after heattreatment in a comparative example.

FIG. 8 is a diagram showing current-voltage characteristics in anelectrode structure obtained in the comparative example.

FIG. 9 is a sectional TEM image of an interface vicinity after heattreatment in a second embodiment.

FIG. 10 is a diagram showing current-voltage characteristics in theelectrode structure obtained in the second embodiment.

EXPLANATION OF REFERENCE SYMBOLS

-   -   10 multilayer structure    -   101 silicon substrate    -   102 silicon dioxide layer (SiO₂)    -   103 indium-gallium-zinc (IGZO) oxide semiconductor layer    -   104 copper-manganese alloy layer    -   105 silicon dioxide layer (SiO₂)    -   106 intermediate layer    -   106 a first region of intermediate layer    -   106 b second region of intermediate layer    -   107 copper layer, electrode body, formed of copper-manganese        alloy layer    -   108 pure copper layer    -   203 indium-gallium-zinc oxide semiconductor layer, IGZO layer    -   206 intermediate layer    -   206 a first region of intermediate layer    -   206 b second region of intermediate layer    -   207 titanium layer

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, an ohmic electrode is formed on an oxidesemiconductor composed of an electrically conductive oxide semiconductormaterial. Examples of the oxide semiconductor layer provided with theohmic electrode include layers (indium-containing oxide layers) composedof zinc oxide (Zn0), indium-tin composite oxide, or other such oxidecontaining indium (element symbol: In). In the present invention, theohmic electrode is in particular located on a layer ofindium-gallium-zinc complex oxide (where the chemical compositionformula is InαGaβZnγOδ: α, β, γ and δ indicate composition, and α>0,β≧0, γ≧0, and δ>0.

In the present invention, a configuration is defined to provide an ohmicelectrode on an indium-containing oxide semiconductor layer so as tosandwich an intermediate layer composed of a first and second regions.Preferably, the first region of the intermediate layer is, so to speak,an indium-rich region in which indium is made present at highconcentration. The indium-concentrated region is defined to be one thatcontains more indium than an interior of the indium-containing oxide.

The intermediate layer is best given a configuration that places theindium-concentrated first region in contact with the indium-rich oxidesemiconductor layer and places the second region in contact with theohmic electrode. As the indium-concentrated first region acts as ahigh-conductivity layer, it is possible by arranging the first andsecond regions in this way to contribute to realize an ohmic electrodeof low electrical contact resistance.

Now, in order to inhibit intrusion into the ohmic electrode of elementsconstituting the indium-containing oxide semiconductor layer,particularly indium, the second region of the intermediate layer is madea region lower in atomic concentration of indium than the first region.If the indium concentration inside the second region is reduced, theeffect of reducing the amount of indium entering the ohmic electrode isenhanced, whereby it is possible to help realize an ohmic electrode lowin impurities and low in electrical resistance. For example, an ohmicelectrode composed of copper containing almost no indium can befurnished.

If the second region is constituted of an oxide containing as its mainelement a metal whose standard free energy of formation of oxide (freeenergy of oxide formation) is of lower value than that of indium, themetal reduces the indium-containing oxide semiconductor layer to becomea metallic oxide, so that indium can be rich in the first region in thecourse of the metallic oxide formation. Simultaneously, the first regionincreases carrier density owing to formation of more oxygen vacanciesthan in the indium-containing oxide. Therefore, the electricalconductivity of the first region improves to help reducing theinterfacial contact resistance with the ohmic electrode. In addition,interfacial adhesion between the ohmic electrode and theindium-containing oxide semiconductor can be improved as a result of aoccurrence of an interfacial reaction. Here, the fact that the value ofthe free energy of oxide formation is negative and the free energy ofoxide formation is low indicates that the absolute value is high andstability as an oxide is high.

In addition, when the second region is constituted of an oxidecontaining as its main element a metal having a lower value of freeenergy of oxide formation than indium, an oxide more stable oxide thanthe indium-containing oxide can be formed. Therefore, elementsconstituting the indium-containing oxide cannot diffusively infiltratethe metallic oxide formed in the second region, so that diffusion ofelements constituting the indium-containing oxide into the ohmicelectrode can also be prevented. As a result, the resistance of theelectrode material can be prevented from increasing unnecessarily.

According to an Ellingham diagram (see, for example, MetallurgicalThermodynamics of Iron, Masayoshi OHTANI, Nikkan Kogyo Shimbun, p 103),metals having a lower value of free energy of oxide formation thanindium include manganese (Mn), molybdenum (Mo), vanadium (V), tantalum(Ta), titanium (Ti), cerium (Ce), aluminum (Al), magnesium (Mg), lithium(Li), calcium (Ca) and so on. Among these metals, manganese (Mn),molybdenum (Mo), vanadium (V) and titanium (Ti) are especiallypreferable. Mn, Mo, V and Ti are all transition metals and can takemultiple valence states. The charge distribution of the regions as awhole can therefore be maintained in the most stable state byappropriately varying the valences in the first region and the secondregion. As a result, the first region and the second region can bothassume a stable state both electrically and thermally, so that long-termreliability can be ensured for both the ohmic characteristics and theadhesion properties.

And even among these transition metals, Mn is particularly preferable.Mn tends to form a solid solution with the Cu that is the mainconstituent element of the ohmic electrode present adjacently.Therefore, when an oxide containing Mn is formed in the second region,there is the advantage of being able to obtain excellent adhesion withrespect to the ohmic electrode.

Like Mn, Ti is also preferable. Ti tends to form an intermetalliccompound with the Cu that is the main constituent element of the ohmicelectrode present adjacently. Therefore, when an oxide containing Ti isformed in the second region, there is the advantage of being able toobtain excellent adhesion with respect to the ohmic electrode.

When the first and second regions contain Mn, it is advantageous to makethe manganese present so as to increase the valence (electrovalence) ofthe manganese from the portion of the second region in contact with theohmic electrode toward the portion of the first region in contact withthe indium-containing oxide semiconductor layer. According to anEllingham diagram, Mn oxide decreases in free energy of oxide formationto become stably present in proportion as the valence of the Mn islower. Therefore, mutual diffusion between the ohmic electrode and theindium-containing oxide semiconductor can be prevented by forming thesecond region of structurally stable Mn oxide. Further, in the firstregion, it is possible by increasing the valence of the Mn to correctdisturbances in the charge distribution of the region reduced andincreased in oxygen vacancies, and to establish an electrically stablestructure.

With Ti, as with Mn, when the first and second regions contain Ti, it isadvantageous to make the Ti present so as to increase the valence of theTi from the portion of the second region in contact with the ohmicelectrode toward the portion of the first region in contact with theindium-containing oxide semiconductor layer. According to the Ellinghamdiagram, a Ti oxide decreases free energy of oxide formation to becomestably present in proportion as the valence of the Ti is lower.Therefore, mutual diffusion between the ohmic electrode and theindium-containing oxide semiconductor can be prevented by forming thesecond region of structurally stable Ti oxide. Further, in the firstregion, it is possible by increasing the valence of the Ti to correctdisturbances in the charge distribution of the region reduced andincreased in oxygen vacancies, and to establish an electrically stablestructure.

It is best to constitute the second region of the intermediate layerfrom an amorphous layer. Elements constituting the oxide semiconductorlayer diffuse and move along grain boundaries at high speed. As anamorphous layer has no grain boundaries, it is possible by constitutingthe second region from an amorphous layer to inhibit diffusive intrusioninto the ohmic electrode of indium and other elements constituting theindium-containing oxide semiconductor layer, whereby it is possible tocontribute to realize an ohmic electrode of low electrical resistance.

As constituting the first region of the intermediate layer from crystalgrains containing indium forms a high conductivity portion, it has theeffect of reducing the electrical resistance of the intermediate layerand lowering the interfacial contact resistance between the ohmicelectrode and the indium-containing oxide semiconductor layer. It istherefore suitable in providing, for example, a thin-film transistor orthe like that utilizes as the active layer an oxide semiconductor layerof high electron mobility that provides high-speed operation. Thecrystal grains containing indium here are nearly spherical or other suchcrystal grains composed of indium. Or they are crystal grains composedof an oxide containing indium.

The thicknesses of the first and second regions forming the intermediatelayer are substantially equal, and the thickness of the intermediatelayer of the combined first and second regions is preferably 3 nm orgreater and 30 nm or less. In the intermediate layer, should theindium-rich first region be much thicker than the second region, thefirst region of high conductivity owing to the rich indium would accountfor a greater proportion of the intermediate layer, so that a resultingelectrode excellent in ohmic properties would be expected. In this case,however, the amount of indium and the like contained in the first regionthat infiltrates into the interior of the electrode would increase as aresult of the second region becoming thinner, so that formation of anelectrode excellent in ohmic properties might sometimes be impeded.Since opportunities for mutual dispersion increase, the overallintermediate layer thickness that is the sum of the individualthicknesses of the first and second regions needs to be at least 3 nm.On the other hand, even if the respective thicknesses of the first andsecond regions are made equal, expanding the overall intermediate layerthickness to greater than 30 nm increases the electrical resistance ofthe intermediate layer, so that an electrode excellent in ohmicproperties is not realized. Therefore, the thickness of the intermediatelayer is preferably 30 nm or less.

Formation on the indium-containing oxide semiconductor layer of anindium-rich first region containing indium at a higher indiumconcentration than that of the indium-containing oxide semiconductorlayer is performed by, for example, forming an oxide gallium-indium-zinc(abbreviation: IGZO) semiconductor layer by the high-frequencysputtering method, and then forming the first region by simultaneoussputtering using an indium oxide (chemical composition formula In₂O₃) orother oxide target that makes the indium content ratio higher than thatof IGZO and a target composed of, for example, manganese. In the case offormation by the high-frequency sputtering method, the thickness of thefirst region is regulated by the length of the sputtering time under thedeposition conditions that determine the predetermined deposition rate.Following this, the second region containing manganese or other metal isformed on the first region by an ordinary vacuum deposition method,electron beam deposition method, high-frequency sputtering method or thelike. If the thickness of the second region is made the same as that ofthe first region, an intermediate layer composed of first and secondregions of the same thickness can be formed.

Further, as another method of forming the indium-concentrated firstregion on the IGZO semiconductor layer, a method of adding metallicindium is available. For example, at the time of forming a manganeseoxide film by the high-frequency sputtering method, metallic indiumdoping is performed concurrently to form the first region. Next, a layerof lower free energy of oxide formation than indium, e.g., of an oxideof manganese (MnxOy: x>0, y>0), is formed on the first region as thesecond region. If the thicknesses of the first and second regions aremade the same, the intermediate layer can be advantageously formed.

In the present invention, the ohmic electrode is formed from copper,which is lower in electrical resistance than aluminum. An ohmicelectrode composed of copper (sometimes abbreviated to copper ohmicelectrode) can be formed by using pure copper or a copper alloycontaining a metal element as an additive element as starting material.The copper alloy for forming the copper ohmic electrode is desirablyadded with a transition metal low in free energy of oxide formation thatcan assume multiple different valences. These are, for example,manganese (element symbol: Mn), molybdenum (Mo), vanadium (V), tantalum(Ta), titanium (Ti), and cerium (Ce). A copper alloy that, among these,uses Mn or Ti as the additive element is particularly preferable as thestarting material.

In order to reduce interconnection resistance and realize good ohmiccontact, it is possible to deposit a film of Mn or Ti on the surface ofthe IGZO semiconductor layer and thereafter complete the electrode bydepositing a pure Cu film. In this case, the Mn or Ti directly contactsthe IGZO semiconductor layer, so that the same effect as above can berealized by heat treatment at a low temperature for a short time.

In addition, with the aim of reducing interconnection resistance, it ispossible to deposit a film of Cu—Mn alloy or Cu—Ti alloy on the surfaceof the IGZO semiconductor layer and thereafter complete the electrode bydepositing a pure Cu film.

After the copper alloy film containing these additive elements has beendeposited on the oxide semiconductor layer, if heat treatment isperformed on the copper alloy, an oxide of the additive element can beformed at the bonding interface between the oxide semiconductor layerand the copper alloy film. This is because the heat treatment diffusesthe additive element toward the oxide semiconductor layer, and theadditive element of lower free energy of oxide formation than the copperpreferentially combines with the oxygen that is a constituent of theoxide semiconductor layer. The layer composed of the oxide of thisadditive element can be advantageously utilized to constitute the secondregion. Concomitantly, the region of the oxide semiconductor reduced bythe additive element becomes oxygen deficient, thereby increasingcarrier density to become a first region having high electricalconductivity. In addition, the interior of the copper alloy from whichthe additive element has escaped comes to consist almost entirely ofpure copper, so that an ohmic electrode can be favorably formed fromcopper of low electrical resistance.

Thus, the additive element can be driven out of the copper alloy by theheat treatment, but to what degree strongly depends on the heattreatment conditions. Depending on the conditions, the additive elementmay in some cases persist in the copper alloy and in other cases betotally driven out of the copper alloy to make the interior pure copper.In the present invention, both states are called “copper alloy.” Forexample, in the case of a Cu—Mn alloy, the state is termed withreference to the pre-heat-treated state as “Cu—Mn alloy” or“copper-manganese alloy” irrespective of the internal Mn concentrationafter heat treatment.

In particular, when a copper-manganese alloy containing manganese as theadditive element at an atomic concentration of a ratio of 1 atomic % to10 atomic % is used as the starting material, a barrier layer thatcontains concentrated manganese of the alloy to be served as a secondregion can be spontaneously formed. This barrier layer acts as adiffusion barrier for preventing oxygen contained inside the oxidesemiconductor layer from penetrating the copper of the electrode. Owingto this effect, the copper of the electrode avoids oxidation, so that anohmic electrode can be constituted from pure copper of low electricalresistance.

After the copper-manganese alloy has been deposited on the oxidesemiconductor layer, the heating temperature for simultaneously formingthe indium-rich first region and the manganese-containing second regionusing the copper-manganese alloy as starting material is 100° C. or more450° C. or less, and the heating time is 5 min or more to 90 min orless. The time period of the heating should be shortened in proportionas the temperature is higher.

First Embodiment

The detail of the present invention will be explained taking as anexample case of formation on a conductive n-type indium-containing oxidesemiconductor layer that uses copper-manganese as the electrodematerial. FIG. 1 is a schematic sectional view of the multilayerstructure of the first embodiment prior to heat treatment. A 50 nm thicksilicon dioxide (SiO₂) insulation layer 102 was formed on a siliconsubstrate 101 using tetraethoxy silicon (abbreviation: TEOS) as startingmaterial. An oxide semiconductor layer (IGZO layer) 103 composed ofn-type amorphous indium-gallium-zinc oxide (a-InGaZnO₄) was deposited onthe SiO₂ insulation layer 102 to a thickness of 30 nm by the ordinaryhigh-frequency sputtering method. The IGZO layer 103 was deposited in amixed atmosphere of argon and oxygen whose volume percentage of oxygenwas 5%, at a pressure of 0.1 pascal (pressure unit: Pa). Next, acopper-manganese alloy layer 104 was deposited on the surface of theIGZO layer 103 by the ordinary high-frequency sputtering method using asthe target material a copper-manganese alloy containing manganese at anatomic concentration of 4%. The thickness of the copper-manganese alloylayer 104 was made 100 nm. In addition, a SiO₂ layer 105 was formed to athickness of 200 nm as an anti-oxidation film by the ordinaryhigh-frequency sputtering method. This completed the formation of amultilayer structure 10. The atomic concentration of the manganeseinside the deposited copper-manganese alloy layer 104 was estimated tobe substantially the same as the atomic concentration of the manganeseof the target material.

Next, the multilayer structure 10 was heated at 250° C. for 60 min inthe atmosphere. Owing to this heating, the manganese inside thecopper-manganese alloy layer 104 diffused and moved to the IGZO layer103 side. The copper-manganese alloy layer 104 therefore became a layer(electrode body) 107 composed substantially of pure copper. Themultilayer structure was left to cool in the aforesaid atmosphere,whereafter a cross-sectional TEM image of the interface region betweenthe electrode body 107 and the IGZO layer 103 was taken. Thecross-sectional TEM image is shown in FIG. 2. The IGZO layer 103 and tworegions 106 a, 106 b differing in contrast were observed to be present.The first region 106 a was observed as a granularly distributedstructure adjacent to the IGZO layer 103. The high-resolution TEM imageobtained from this portion exhibited a periodic lattice pattern,confirming that it was crystalline. The second region 106 b was observedas a layer adjacent to the electrode body 107 (ohmic electrode) that hadcontinuous weak contrast. The contrast did not change when the specimenwas rotated and no periodic lattice pattern was observed in thehigh-resolution TEM image, demonstrating that the second region 106 bwas amorphous. In addition, the thicknesses of the first region 106 aand second region 106 b were substantially the same, and the thicknessof the intermediate layer 106 composed of the first region 106 a andsecond region 106 b was 6 nm.

Analysis by EDX (Energy-Dispersive X-ray microanalysis) was performedalong the direction perpendicular to the layer interface (intermediatelayer 106) shown in FIG. 2, and the results obtained for thedistributions of the X-ray intensities emitted by the respectiveconstituent elements are shown in FIG. 3. The X-ray intensitydistributions correspond to the concentration distributions of therespective elements, making it possible to ascertain the relativechanges among the element concentrations with position. The first region106 a in contact with the IGZO layer 103 is shown to be a regionabundantly containing elements constituting the IGZO layer 103,particularly indium. The atomic concentration of indium inside the firstregion 106 a (about 35 as X-ray intensity) was about 1.4 times as highas the atomic concentration of indium inside the IGZO layer 103 (about25 as X-ray intensity). Further, as shown in FIG. 3, the interior of thefirst region 106 a contained oxygen, and the atomic concentrationthereof was lower than the atomic concentration of oxygen inside theIGZO layer 103. Therefore, the first region 106 a was judged to be anindium-rich oxide layer.

Further, the second region 106 b present in contact with the electrodebody 107 was, from the results of FIG. 3, a layer containing manganeseas the main constituent element. The atomic concentration inside thesecond region 106 b of indium and other elements constituting the IGZOlayer 103 was low in comparison with the first region 106 a, but copperwas contained at higher concentration than in the first region 106 a. Inother words, the second region 106 b had a preferable structuralconfiguration as a barrier layer for preventing mutual diffusion ofcopper that was the main constituent element of the electrode body 107and the IGZO layer 103. The second region 106 b had a thickness of a fewnm and was a thin layer of the same thickness as the first region 106 a.

The results of analysis by EELS (Electron Energy-Loss Spectrocopy) alongthe direction perpendicular to the layer interface (intermediate layer106) of FIG. 2 are shown in FIG. 4. FIG. 4 is a diagram showing theobserved intensities of copper (Cu), manganese (Mn) and oxygen (O)obtained by the ELLS method. Starting from the left are shown theelectrode body 107, second region 106 b, first region 106 a, and IGZOlayer 103. As a result of the heat treatment, Mn concentration in theelectrode body 107 is so low as to be negligible. Further, Mnconcentration is highest in the second region 106 b. The first region106 a showed flat Mn intensity distribution, suggesting formation of aMn-containing equilibrium layer. Further, oxygen intensity is weaker inthe first and second regions 106 a, 106 b than inside the IGZO layer103. Considered in terms of oxygen concentration, this indicates thatoxygen concentration is lower in the first and second regions 106 a, 106b than inside the IGZO layer 103 and that oxygen vacancies occurred inthe first region 106 a together with Mn reduced the IGZO layer 103 toform Mn oxide in the intermediate layer 106.

The chemical state of presence of the manganese contained in the secondregion 106 b was identified by the X-ray photoelectron spectroscopymethod (abbreviation: XPS) method. FIG. 5 shows the binding energybetween the manganese of the second region 106 b and the oxides of theindium, gallium and zinc of the IGZO layer 103. Manganese contained inthe second region 106 b was present in the state of oxide (Mn_(x)O_(y):0<x, 0<y), and the binding energy of manganese near the surface of thesecond region 106 b was measured to be 640.5 electron volts (energyunit: eV). The binding energy of manganese at the center of the secondregion 106 b was 641.5 eV and gradually increased to 641.7 eV on thefirst region 106 a side. From this it was learned that going from thesecond region 106 b toward the first region 106 a manganese was presentin chemical states that changed from manganese oxide (MnO) throughmanganese trioxide (Mn₂O₃) to manganese dioxide (MnO₂). In other words,going from the second region 106 b toward the first region 106 a,manganese was present with valance (electrovalence) increasing from 2(in the case of MnO) to 4 (in the case of MnO₂).

On the other hand, in the region from the intermediate layer 106 acrossthe IGZO layer 103, the bonding energies of indium, gallium and zincwere 445.1 eV, 1119.4 eV and 1023.1 eV, respectively. From this it wasjudged that indium, gallium and zinc were respectively present in thecombined forms of indium trioxide (In₂O₃), gallium trioxide (Ga₂O₃) andzinc oxide (ZnO). Further, in contrast with the case of manganese, nochange was observed in the bonding energies of indium, gallium and zincfrom the intermediate layer 106 across the IGZO layer 103. In otherwords, no change was brought about in the indium, gallium and zincvalances in the depth direction.

With respect to the electrode body 107 that came to be formed of nearlypure copper owing to the diffusion by the aforesaid heating of manganesethat helped to form the second region 106 b, rectangular electrodes werefabricated to a width of 120 micrometers (length unit: μm) and length(length in the direction parallel to current flow direction) of 60 μmand direct current was then passed between opposing electrodes. Thecurrent (I)-voltage (V) characteristics in this case are shown in FIG.6. The numerals in the drawing are distances between the electrodes inthe measurement. In the case of using copper-manganese alloy as thestarting material, the results obtained exhibited good ohmiccharacteristics of linear current increase relative to voltage from lowapplied voltage. In addition, electrical contact resistance was measuredby the TLM (Transmission Line Mode) method. The contact resistance atroom temperature of ohmic electrodes formed by using a copper alloycontaining manganese at the rate of 4% in terms of atomic concentrationwas calculated to be 1.2 to 29×10⁻⁴Ω·cm². Moreover, the mobility of theamorphous IGZO layer after forming the copper ohmic electrodes wasmaintained high, at about 7 to 8 cm²/V·s.

When, in order to investigate adhesion, Scotch Tape was attached to andpeeled off the copper film surface, nothing adhered to the attachedsurface of the tape and no film detachment occurred. From this, it wasclear that adhesion between the copper-manganese alloy (electrode body107) and the IGZO layer 103 is excellent.

As the electrode structure, it is possible to use one obtained byadhering a copper/manganese two-layer film or a copper/copper-manganesealloy two-layer film to the IGZO layer and applying heat treatment.These also make it possible to obtain the same results as one obtainedby applying heat treatment to the aforesaid copper-manganese alloy/IGZOlayer.

Comparative Example

The copper-manganese alloy of the first embodiment was replaced withpure copper (99.9999% purity) and a test was performed under the sameconditions. The results observed by cross-sectional TEM of the interfacevicinity of the copper (108) and the IGZO (103) are shown in FIG. 7.Even though heat treatment was conducted at 250° C. for 1 h, formationof an intermediate layer by an interfacial reaction was not confirmed.Further, when, in order to investigate adhesion, Scotch Tape wasattached to and peeled off the copper thin film surface, copper thenfilm adhered to the attached surface of the Scotch Tape, revealing thatadhesion between the copper and IGZO was deficient. Further, the resultswhen an electrode array was formed and current-voltage was measuredbetween electrodes are shown in FIG. 8. It is indicated that non-linearrelationships were obtained, the slopes were moderate compared withthose for the copper-manganese alloy shown in FIG. 5, and theinterfacial contact resistance was high.

Second Embodiment

The copper-manganese alloy in the embodiment set out above was changedto titanium and a test was performed under the same conditions. Theresults observed by TEM of the interface vicinity of the titanium andthe IGZO are shown in FIG. 9. As heat treatment was conducted at 250° C.for 1 h, the Ti layer 207 and IGZO layer 203 reacted to form anintermediate layer 206. Similarly to the case of the copper-manganesealloy, this intermediate layer 206 was composed of a first region 206 aand a second region 206 b. It can be seen that the first region 206 awas composed of fine crystal grains and the second region 206 b wascomposed of amorphous material exhibiting uniform contrast.

A titanium electrode array was formed by using the lift-off method andthe current-voltage relationship between electrodes was measured. Theresults are shown in FIG. 10. The numerals in the drawing indicate thedistances between the electrodes used in the measurement. Distinctlinear relationships were obtained at every electrode spacing, and theinterface can be seen to be ohmic contact. Electrical contact resistancewas determined by the TLM method and found to be 0.2 to 1.5×10⁻⁴Ω·cm²,indicating performance on a par with copper-manganese.

Like results can also be achieved using a copper/titanium two-layerfilm, copper-titanium alloy film, or a copper/copper-titanium two-layerfilm as the electrode material structure.

INDUSTRIAL APPLICABILITY

The ohmic electrode composed of copper of the present invention can beused as, for example, the source or drain electrode of a thin-filmtransistor with an active layer of indium-containing oxidesemiconductor. The layer is formed from n-type gallium-indium-zinc oxideor the like. For example, it is formed of n-type. In particular, it canbe favorably formed from gallium-indium-zinc composite oxide, which hasa high electron mobility about ten times higher than that of theamorphous silicon (electron mobility of about 0.3 to about 1.0 cm²/V·s)that is the conventional active layer material.

Further, in the semiconductor device known as a light-emitting diode(abbreviation: LED), for example, it can be used as an n-type or p-typeohmic electrode provided at the so-called window layer composed ofoptically transparent oxide semiconductor for efficiently transmittingemitted light to the exterior. For example, it can be used as an ohmicelectrode provided at the window composed of indium-tin (element symbol:Sn) composite oxide or indium-zinc (element symbol: Zn) composite oxideused in a pn junction double heterostructure (abbreviation: DH) LED witha light-emitting layer of gallium-indium-nitride (GaxInyN: 0≦x, y≦1,x+y=1).

1. A semiconductor device comprising: a semiconductor layer composed ofan oxide semiconductor material containing indium; an ohmic electrodeprovided on the semiconductor layer and having an ohmic contact with thesemiconductor layer; and an intermediate layer provided between thesemiconductor layer and the ohmic electrode, wherein the intermediatelayer includes a first region whose indium atomic concentration isgreater than that of an interior of the semiconductor layer and a secondregion whose indium atomic concentration is less than that of the firstregion.
 2. The semiconductor device as recited in claim 1, wherein thefirst region is located in contact with the semiconductor layer and thesecond region is located in contact with the ohmic electrode.
 3. Thesemiconductor device as recited in claim 1, wherein the first region iscomposed of crystal grains containing indium (In).
 4. The semiconductordevice as recited in claim 1, wherein the second region is composed ofamorphous material.
 5. The semiconductor device as recited in claim 1,wherein the thickness of the intermediate layer where the first regionand the second region meet is 3 nm or more and 30 nm or less.
 6. Thesemiconductor device as recited in claim 1, wherein the first and secondregions are constituted to contain an oxide of a metal forming the ohmicelectrode.
 7. The semiconductor device as recited in claim 6, whereinthe metal forming the ohmic electrode is a metal with a free energy ofoxide formation of a lower value than indium.
 8. The semiconductordevice as recited in claim 6, wherein the metal forming the ohmicelectrode contains at least one among manganese (Mn), molybdenum (Mo),and titanium (Ti).
 9. The semiconductor device as recited in claim 8,wherein the metal forming the ohmic electrode contains manganese. 10.The semiconductor device as recited in claim 9, wherein anelectrovalence of the manganese increases from the second region towardthe first region.
 11. The semiconductor device as recited in claim 9,wherein an atomic concentration of the manganese exhibits maximum valuein the second region and is lower in the first region than in the secondregion.
 12. The semiconductor device as recited in claim 8, wherein themetal forming the ohmic electrode contains titanium.
 13. Thesemiconductor device as recited in claim 12, wherein the electrovalenceof the titanium increases from the second region toward the firstregion.
 14. The semiconductor device as recited in claim 1, whereinoxygen concentrations of the first and second regions are lower than anoxygen concentration of the semiconductor layer.
 15. The semiconductordevice recited in claim 1, wherein the ohmic electrode is formed of acopper alloy whose main constituent element is copper.
 16. Thesemiconductor device as recited in claim 15, wherein the second regioncontains more copper than the first region.
 17. The semiconductor deviceas recited in claim 2, wherein the first region is composed of crystalgrains containing indium (In).